1. Technical Field
This invention relates generally to input and output scanners, and more particularly to a scanner having a pixel clock that compensates for scanner non-linearity.
2. Background Information
A scanner includes some type of scanning means for directing a light beam to a spot on a surface to be scanned. It does so in such a way that the spot moves across the surface along a scan line in a precisely controlled scan cycle. That enables various input and output functions such as reading a document or printing a page.
Scanner non-linearity refers to variations in spot velocity occurring as the spot moves along the scan line during the scan cycle. It is typically caused in such systems as polygon or galvanometer laser scanner systems by system geometry or a velocity variation of the scanning means and it can affect scanner performance. A scanner having a multifaceted rotating polygon, for example, directs the light beam at a constant angular velocity. But the spot is farther from the polygon facets at the ends of the scan line than it is at the center and so spot velocity increases as the spot moves from the center toward the ends.
Some scanners compensate for such non-linearity electronically using a variable frequency pixel clock (sometimes called a scanning clock). The pixel clock produces a pulse train (i.e., a PIXEL CLOCK signal) that is used to turn the light beam on and off at each pixel position along the scan line. Varying the clock frequency and thereby the timing of individual pulses in the pulse train serves to control pixel placement along the scan line. That is done according to variations in spot velocity in order to more evenly space the pixels and thereby at least partially compensate for what is sometimes called pixel position distortion (i.e., uneven pixel spacing caused by scanner non-linearity). The technique can even be used to introduce distortion where desired or to compensate for distortion intrinsic to input data.
One existing pixel clock includes a voltage controlled oscillator (VCO) in a phase locked loop (PPL). The output of the VCO provides the basic PIXEL CLOCK signal and the phase locked loop is locked to reference pulses produced by what is sometimes called a grid clock. So called for the accurately ruled grating or grid employed, the grid clock includes suitable componentry for causing a second or reference light beam to be directed by the multifaceted rotating polygon or other scanning means toward the grid in such a way that the second light beam moves along the grid as the primary or scanning light beam moves along the scan line.
Meanwhile, alternating clear and opaque areas on the grid (or reflecting and non-reflecting areas) modulate the second beam. To better visualize the process, consider a grid used with a nine inch scan line. Such a grid may include, for example, a series of 301 parallel lines etched at 0.03 inch intervals on a ten to twelve inch strip of glass. Thus, the grid extends nine inches like the scan line and as the second beam moves across the grid, the etched lines periodically block passage of light through the glass. So detector circuitry collecting and processing light on the far side of the glass produces a series of 300 reference pulses that provide accurate information about spot velocity and position along the scan line, each reference pulse representing an increment of spot movement along the scan line.
Locking the phase locked loop to the reference pulses controls the number of clock pulses produced for each of the scan cycles. In addition, it compensates somewhat for scanner non-linearity by forcing phase coincidence between each of the reference pulses and a corresponding one of the clock pulses. That causes the clock frequency to vary in a way that approximates the ideal case in which the clock frequency is proportional to spot velocity (which varies according to spot position along the scan line). A large number of reference pulses per scan cycle improves the approximation. In that regard, the grid clock described above, produces 300 reference pulses per scan cycle. For a typical resolution of 300 pixels or dots-per-inch (dpi) and a nine inch long scan line, that translates to nine pixels per reference pulse.
One drawback in using a grid clock of the type described is the cost and complexity. The fine grid pattern, the need to image the reference beam to a high resolution spot on the grid, and the required light collection optics all contribute. In addition, local imperfections on the grid can cause significant phase jitter that limits usefulness of the design. So it is desirable to have a better way to control the pixel clock.
U.S. Pat. No. 4,729,617 describes a scanning clock generating device that includes a voltage controlled oscillator in a phase locked loop. The PLL is locked to reference pulses produced by a position control clock instead of a grid clock, and so some grid clock problems are avoided in the process of approximating the ideal frequency variation curve 40 shown in FIG. 3 of that patent. One problem with the device, however, is that it works for only one scanning speed, requiring programming modifications along with low pass filter and other circuit changes to accommodate a two speed scanner, for example. In addition, the PLL is configured as a control system in which a constant rate of change of the controlled variable requires a constant error signal under steady state conditions (i.e., a type 1 control system). Instead of forcing phase coincidence it actually requires a phase error. That can impair accuracy. So it is desirable to have a way to control the pixel clock that overcomes those problems as well.